11 wimpy galaxies are beating up our brightest explanation yet for a deep, dark cosmic mystery
People have searched for tiny, hypothetical particles of dark matter for years in machines deep below the ground and in space, but we have yet to find a single speck. All we know is that it tugs on "normal" matter with gravity.
That's why some scientists are revisiting an old idea that black holes born at the dawn of time - invisible monsters that could have avoided our detection until now - are a prime suspect for all that missing mass.
Researchers call these speculative black holes "massive compact halo objects," or MACHOs, since dark matter lurks in a "halo" in and around big galaxies.
But a new study in The Astrophysical Journal Letters offers a serious reality check to the idea that MACHOs are dark matter.
Timothy Brandt, an astrophysicist at the Institute for Advanced Study, wrote the study after he took a close look at 11 dim, feeble, and weird little galaxies. The stars there are choking with dark matter, at least compared to larger galaxies like the Milky Way or Andromeda, but the little galaxies don't seem to show any obvious signs of harboring a flotilla of old black holes.
"These galaxies would be less dense and larger than we see," Brandt told Business Insider.
Instead they're inexplicably compact. And that could represent a big threat to the MACHO hypothesis.
The mystery of 11 non-puffy dwarfs
Brandt's interest started with relatively recent discoveries of small, seemingly very old, and ultra-faint star clusters.
Some of these clusters are large enough to be called dwarf galaxies, yet are billions of times dimmer than larger galaxies. Such ultra-faint dwarfs are thought to be incredibly common, Brandt says, perhaps outnumbering visible galaxies by 10- or 100-to-one.
Nearly all of their young, bright stars have burned out over the eons, which is why these ultra-faint dwarf galaxies are so hard to find.
"If you're looking with your own eyes you'd never see them," Brandt said, noting they can be as small as a few thousand stars. "We've only been finding these things for about 10 years with computer algorithms."
What made them so interesting to Brandt is that some of these clusters exist at all.
By mass our own Milky Way is somewhere between 70% and 80% dark matter - most of it lurking near the outer edge of the galaxy. Meanwhile, ultra-faint dwarf galaxies are roughly 99% dark matter.
"The dark matter is holding them together and preventing them from flying apart," Brandt told Business Insider.
And that's where Brandt realized he could see if a bunch of old black holes between 20 to 100 times the mass of the sun (a size range for MACHOs that has yet to be ruled out) really exist there. If so, they'd accelerate stars as they passed nearby, causing the entire cluster or galaxy to "puff" outward over billions of years.
A diffuse cloud of dark matter particles, on the other hand, would keep the cluster glued together.
Eri II is a 3- to 12-billion-year-old dwarf galaxy that astronomers barely and recently noticed hanging out near our much-larger Milky Way galaxy.
Brandt's calculations show that if the cluster's dark matter is made of MACHOs, then it would have puffed out and faded into Eri II a long time ago.
But there it was: A small, dim, ancient star cluster that was much too compact, as well as its parent galaxy.
He noted the 10 dwarf galaxies that are similar to Eri II should act the same, but that "we don't see puffy ultra-faint dwarf galaxies."
"This one cluster could be a coincidence. But if you look at all the other dwarf galaxies, I don't see how [the dark matter] could be black holes between 20 and 100 solar masses," Brandt said.
Why dark matter might be ancient black holes
Brandt's study comes at a time when scientists like Alexander Kashlinsky, a cosmologist at NASA, have started revisiting the MACHO idea that dark matter may just be a bunch of really, really old black holes.
That's because several searches for tiny bits of dark matter, called "weakly interactive massive particles" or WIMPs, keep striking out.
"[WIMP] searches have been going on for quite some time, but nothing has been reliably detected, slowly shrinking the box of parameters where [dark matter] particles can hide," Kashlinsky previously wrote in an email to Business Insider.
Meanwhile, MACHOs of a few sizes - including those weighing roughly 30 solar masses - have yet to be firmly ruled out.
Scientists figured MACHOs probably formed within a fraction of a second of the Big Bang some 13.8 billion years ago. This was right as the event's hot, energetic soup of plasma cooled into the first subatomic bits of matter (then later into atoms, gases, stars, and so on).
These primordial black holes would have lurked in the background as the first stars, galaxies, and planets started to form millions of years later, subtly influencing their destinies while vastly outweighing them; for every bright, shining star, four stars' worth of mass was forever locked away in a black hole.
Trouble is, no one could find enough black holes to finger them as dark matter. So this spooky hypothesis lost steam over the past couple of decades.
LIGO's first-ever detection of gravitational waves was made possible by two black holes smacking together, each roughly 30 times the mass of our sun.
Thing is, this size of black hole is supposed to be rare in nature - so detecting two at once is rather remarkable.
Calculations suggest black holes of that size could be MACHOs that formed within a second inside the hot, energetic soup of the Big Bang.
If so, then a lot more of them are out there, others will eventually collide, and LIGO will keep detecting them as it listens for ripples in space-time.
"We should know the answer within a couple of years," Kashlinsky previously said, later emphasizing that this is irrespective of Brandt's challenging findings.
"LIGO is starting a new run of observations in September. If they keep discovering these beasts at this rate, every one to two weeks, we should have good enough statistics to rule in the possibility."
'We could be in trouble'
Both Brandt and Kashlinsky told Business Insider the ultra-dim dwarf galaxy study isn't the end of the end of the line for MACHOs.
Although unlikely, the star cluster in Eri II may have recently dropped into the center - so its stars may not have had enough time to be accelerated by MACHOs.
"It takes a long time for MACHOs to share their energy with the stars" if they exist, Brandt said.
Kashlinsky says another possibility, as pointed out in Brandt's study, is that a chance alignment makes the star cluster only appear to be near the core of Eri II. If true, the cluster exists near the edge or outside of the dwarf galaxy, which could explain its non-puffiness (there is less mass nearby to pull on stars and help accelerate them).
Brandt's study pegs that possibility at about 1%. "Statistically speaking that is non-negligible at all," Kashlinsky wrote in an email to Business Insider.
A more radical explanation is that MACHOs may be smaller than anyone thought, perhaps somewhere between the the weight of the moon and an asteroid. These may be impossible to detect, though, since Brandt said since they'd be less than a millimeter in size.
A scenario like that is one Brandt fears the most.
"It's possible there is no interaction of dark matter [with normal matter] except through gravity," he said. "If that's the case, we're in trouble. We've never come to that point where we know something is out there but is completely invisible to our experiments."
"But I'm not that pessimistic," he said. "Not yet."